hole or not hole in prepared mesh for 3d printing?
As someone working in 3D printing, I can tell you that those tutorials are wrong, and making a hole inside a mesh to make it 'empty' inside is only going to create troubles.

Basically, to 3D print, your STL file goes through a program called a 'slicer', which will analyse and slice the mesh in layers before sending it to the printer for the actual printing layer by layer.

When the slicer analyses the mesh, it will identify the skin/walls of the mesh, determine what is outside and inside, which will also help it determine if supports are needed or not.

If, for example, you want to print a cube, making the mesh hollow inside means that the slicer will add support for the internal roof of the cube, support that you'll never be able to take out because it's a cube, and you cannot reach inside unless you break the cube open.

Plus, having mesh with 'holes' inside can sometimes confuse the slicer, especially if the walls of the mesh are thin, and prevent it from correctly interpreting what is inside and outside, or mess with the layers.

Besides, making a mesh empty is useless because all slicers have a setting called 'Infill' which determine the shape and percentage of the internal structure. Depending on your mesh, and how confident you are in the material being used, you can set it to 0%, meaning the printer will only print the walls of your mesh, up to 100% if you want to print a solid block.

So, to sum it up, making a hole inside a mesh is a bad idea, it will create problems, it won't really save you money nor time, especially since there is a setting in the slicer that does the same while ensuring proper printing.